It is known from the prior art to utilize rotors having superconducting rotor windings for electric machines such as e.g. synchronous motors and synchronous generators. Such rotors are preferably used in power plant generators for generating electric power, wherein higher power densities, lower losses and additional advantages are achieved by the rotating superconducting winding of the rotor. In order to produce superconduction in the rotor winding in rotors of said kind, said rotors are provided with a cooling tube system in which cryogenic coolant such as e.g. helium, hydrogen, neon or nitrogen circulates automatically. The coolant is caused to circulate by the centrifugal force produced by rotation of the rotor.
Publication DE 103 03 307 A1 describes a rotor with superconducting rotor winding and a cooling system in the form of cooling tube loops which are provided on two radially opposite sides of the rotor. In this arrangement said cooling tube loops are connected to a coolant reservoir. During rotation operation the coolant is conveyed from the reservoir into the cooling tube loops and from there back into the reservoir.
In the case of known rotors with superconducting winding and corresponding cooling system it has been found disadvantageous that when the rotor is not rotating the coolant always flows into the geodetically lower cooling tubes and therefore uniform cooling of the rotor winding cannot be achieved while the rotor is stationary. If the rotor is cooled without rotation, e.g. in a cooling process disposed upstream of normal rotor operation, this results in coolant flowing through at most only half of the cooling tubes, which can lead to undesirable thermally induced stresses. Consequently, it must be ensured that the coolant flows through the entire cooling tube system even outside of normal operation of the rotor. In conventional rotors this can only be achieved by rotating the rotor continuously or at intervals, which, however, makes it necessary to install a drive for the rotor.